Display mode with alpha channel on Symbian devices

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Article Metadata

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Tested with

Devices(s): All (S60)

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Compatibility

Platform(s): S60 3rd Edition
S60 5th Edition

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Article

Keywords: TDisplayMode, EColor16MU, EColor16MA, EColor16MAP

Created: User:Kbwiki (December 10, 2009
Last update: March 23, 2010)

Last edited: hamishwillee (01 Aug 2012)

Overview

Symbian C++ applications should be ready to handle a display mode (TDisplayMode) with the alpha channel defined (EColor16MA and EColor16MAP).

Detailed description

To implement low-level graphics operations, such as conversion from one format (colour depth) to another or drawing directly to the data address of an offscreen bitmap, one must be aware of the display mode (colour depth) and colour format used by the device. Possible display modes are listed in the TDisplayMode enumeration.

For current and upcoming Symbian devices with true colour displays, the following display modes are used:

• EColor16MU    True colour display mode (32 bpp, but top byte is unused and unspecified)

• EColor16MA    Display mode with alpha (32 bpp - 24bpp colour plus 8bpp alpha)

• EColor16MAP   Display mode with alpha (32 bpp - 24bpp colour plus 8bpp alpha) - colour channels pre-multiplied with alpha value

Applications that are designed to run in EColor16MU display mode typically require no modifications to accommodate EColor16MA or EColor16MAP as the default mode used by the screen device. This is because the colour format and bits-per-pixel length are identical in all modes. The only difference is that the top 8 bits unused in EColor16MU are used for the alpha channel in the two other modes. However, this should be taken into account when handling TDisplayMode values in the code.

Solution

To ensure that the application works on devices with the EColor16MU default display mode as well as on devices with the EColor16MA display mode, avoid using the following type of code when handling TDisplayMode values:

  switch( iWindow.DisplayMode() )
    {
    case( EColor4K ) :  { bitsPerPixel = 12; break; }
    case( EColor64K ):  { bitsPerPixel = 16; break; }
    case( EColor16M ):  { bitsPerPixel = 24; break; }
    case( EColor16MU ): { bitsPerPixel = 32; break; }
    default: { Panic( EPanicUnsupportedDisplayMode ); } // Error: Panics if display mode is EColor16MA(P)!
    }

Instead, the code should be written as follows:

  switch( iWindow.DisplayMode() )
    {
    case( EColor4K ) : { bitsPerPixel = 12; break; }
    case( EColor64K ): { bitsPerPixel = 16; break; }
    case( EColor16M ): { bitsPerPixel = 24; break; }
    // Valid for EColor16MU, EColor16MA, EColor16MAP*
    default: bitsPerPixel = 32; 
    }

Similarly, for code that uses display mode value in managing bitmaps (CFbsBitmap) and/or graphics contexts (CFbsBitGc), display modes with alpha channels can be treated as EColor16MU if there's no need for transparency:

 iDisplayMode = iCoeEnv->ScreenDevice()->DisplayMode();
 if ( iDisplayMode == EColor16MA || iDisplayMode == EColor16MAP )
   {
   iDisplayMode = EColor16MU;
   }
 ...

* Note however that to get correct transparency results when directly manipulating pixel data in bitmaps with EColor16MAP mode, each colour component must be premultiplied with the alpha value.